HIV is most commonly acquired by mucosal route during sexual intercourse. Therefore, blocking this means of transmission will rapidly decrease HIV incidence worldwide. In the first days following mucosal exposure HIV must overcome numerous barriers offering a window of opportunity to act before HIV establishes infection. Unfortunately, the critical events that take place after viral exposure are poorly understood. We propose that HSV-2 influences these early events by modulating the mucosal microenvironment and, to explore how, we will use our recently established macaque model of HSV-2 infection in combination with a model of SHIV vaginal transmission. This combination constitutes a new powerful tool to study the earliest stages of SHIV infection in a controlled manner, explore the effect of HSV-2 and identify the events that influence transmission. Interestingly, we found that rectal HSV-2 infection increases the percentage of 47highCD4+ T cells in the site of infection (rectal tissue), blood, and draining lymph nodes. And in earlier wors we described a specific interaction between the HIV/SIV envelope and 47. Mucosal CD4+ T cells expressing high levels of this receptor (47high) constitute a preferential target for HIV infection. Recent data has further indicated that early-transmitted viruses exhibit increased reactivity for 47 (compared to chronic isolates and to their reactivity for CD4), suggesting that this molecule is especially important during the initial stages of infection. Additionally, we foun that dendritic cells (DCs) infected with HSV-2 in-vitro expand the 47high subset of CD4+ T cells in co-culture and increase HIV infection in these mixtures. Thus, we hypothesize that HSV-2 influences DC function within the cervico-vaginal tissue and that this, in turn, increases the presence of 47highCD4+ T cells in the mucosa. We intend to use HSV-2 as a tool to dissect the role of 47highCD4+ T cells during early events of HIV transmission across the cervico-vaginal mucosa. We will study how HSV-2 modulates T cell and DC biology within the cervico-vaginal tissues (cell migration, activation, phenotype) to determine how this influences susceptibility to repeated low-dose SHIVSF162P3 vaginal challenge. Moreover, using a drug that specifically inhibits HIV-envelope binding to 47 versus a mutant virus that has increased affinity for 47 we will further demonstrate the importance of the 47-HIV interaction in the onset of SHIV infection. These studies will provide critical insight into the role of 47 in the earliest moments of HIV infection across the cervico-vaginal mucosa and how this is influenced by HSV-2 infection, opening up novel approaches to tackle HIV spread.